Microscopic structure of semilunar valves of the body will be examined from a perspective of interrelated functions of the parts. Whereas most views of cardiovascular valve function envision the passive movement of valve leaflets in response to blood flow, our theory proposes a dynamic role for the vascular walls in leaflet movement. Previous reports of valve structure have overlooked functional considerations. The manner in which the leaflets and wall are structurally associated, however, determines whether suspected functions can occur and additionally relates to the manner in which the valvular parts respond to the stresses of function. Our theory further supposes that tissues in the valve turn over in response to such repeated stresses. By labeling tissues are regularly replaced and whether particular stresses at any points are reflected in restricted sites of turnover. Information about sites of stress can bear upon the design of artificial valves. BIBLIOGRAPHIC REFERENCES: Brewer, R.J., J.D. Deck, B. Capati and S.P. Nolan 1976 The dynamic aortic root. J. Thor. Cardiovasc. Surg., 72: 413-417. Deck, J.D. and P.J. Schneider 1977 Radioautographic studies of cell and tissue turnover in the aortic valve of rats. Anat. Record, in press (abstract).